Dynamic regulation of inter-organelle communication by ubiquitylation controls skeletal muscle development and disease onset

Ubiquitin-proteasome system (UPS) dysfunction is associated with the pathology of a wide range of human diseases, including myopathies and muscular atrophy. However, the mechanistic understanding of specific components of the regulation of protein turnover during development and disease progression in skeletal muscle is unclear. Mutations in KLHL40, an E3 ubiquitin ligase cullin3 (CUL3) substrate-specific adapter protein, result in severe congenital nemaline myopathy, but the events that initiate the pathology and the mechanism through which it becomes pervasive remain poorly understood. To characterize the KLHL40-regulated ubiquitin-modified proteome during skeletal muscle development and disease onset, we used global, quantitative mass spectrometry-based ubiquitylome and global proteome analyses of klhl40a mutant zebrafish during disease progression. Global proteomics during skeletal muscle development revealed extensive remodeling of functional modules linked with sarcomere formation, energy, biosynthetic metabolic processes, and vesicle trafficking. Combined analysis of klh40 mutant muscle proteome and ubiquitylome identified thin filament proteins, metabolic enzymes, and ER-Golgi vesicle trafficking pathway proteins regulated by ubiquitylation during muscle development. Our studies identified a role for KLHL40 as a regulator of ER-Golgi anterograde trafficking through ubiquitin-mediated protein degradation of secretion-associated Ras-related GTPase1a (Sar1a). In KLHL40 deficient muscle, defects in ER exit site vesicle formation and downstream transport of extracellular cargo proteins result in structural and functional abnormalities. Our work reveals that the muscle proteome is dynamically fine-tuned by ubiquitylation to regulate skeletal muscle development and uncovers new disease mechanisms for therapeutic development in patients.

Electrical impedance myography detects age-related skeletal muscle atrophy in adult zebrafish

Age-related deficits in skeletal muscle function, termed sarcopenia, are due to loss of muscle mass and changes in the intrinsic mechanisms underlying contraction. Sarcopenia is associated with falls, functional decline, and mortality. Electrical impedance myography (EIM)-a minimally invasive, rapid electrophysiological tool-can be applied to animals and humans to monitor muscle health, thereby serving as a biomarker in both preclinical and clinical studies. EIM has been successfully employed in several species; however, the application of EIM to the assessment of zebrafish-a model organism amenable to high-throughput experimentation-has not been reported. Here, we demonstrated differences in EIM measures between the skeletal muscles of young (6 months of age) and aged (33 months of age) zebrafish. For example, EIM phase angle and reactance at 2 kHz showed significantly decreased phase angle (5.3 ± 2.1 versus 10.7 ± 1.5°; p = 0.001) and reactance (89.0 ± 3.9 versus 172.2 ± 54.8 ohms; p = 0.007) in aged versus young animals. Total muscle area, in addition to other morphometric features, was also strongly correlated to EIM 2 kHz phase angle across both groups (r = 0.7133, p = 0.01). Moreover, there was a strong correlation between 2 kHz phase angle and established metrics of zebrafish swimming performance, including turn angle, angular velocity, and lateral motion (r = 0.7253, r = 0.7308, r = 0.7857, respectively, p < 0.01 for all). In addition, the technique was shown to have high reproducibility between repeated measurements with a mean percentage difference of 5.34 ± 1.17% for phase angle. These relationships were also confirmed in a separate replication cohort. Together, these findings establish EIM as a fast, sensitive method for quantifying zebrafish muscle function and quality. Moreover, identifying the abnormalities in the bioelectrical properties of sarcopenic zebrafish provides new opportunities to evaluate potential therapeutics for age-related neuromuscular disorders and to interrogate the disease mechanisms of muscle degeneration.

Tetraspanin CD82 is necessary for muscle stem cell activation and supports dystrophic muscle function

Background

Tetraspanins are a family of proteins known to assemble protein complexes at the cell membrane. They are thought to play diverse cellular functions in tissues by modifying protein-binding partners, thus bringing complexity and diversity in their regulatory networks. Previously, we identified the tetraspanin KAI/CD82 as a prospective marker for human muscle stem cells. CD82 expression appeared decreased in human Duchenne muscular dystrophy (DMD) muscle, suggesting a functional link to muscular dystrophy, yet whether this decrease is a consequence of dystrophic pathology or a compensatory mechanism in an attempt to rescue muscle from degeneration is currently unknown.

Methods

We studied the consequences of loss of CD82 expression in normal and dystrophic skeletal muscle and examined the dysregulation of downstream functions in mice aged up to 1 year.

Results

Expression of CD82 is important to sustain satellite cell activation, as in its absence there is decreased cell proliferation and less efficient repair of injured muscle. Loss of CD82 in dystrophic muscle leads to a worsened phenotype compared to control dystrophic mice, with decreased pulmonary function, myofiber size, and muscle strength. Mechanistically, decreased myofiber size in CD82^−/− dystrophic mice is not due to altered PTEN/AKT signaling, although increased phosphorylation of mTOR at Ser2448 was observed.

Conclusion

Basal CD82 expression is important to dystrophic muscle, as its loss leads to significantly weakened myofibers and impaired muscle function, accompanied by decreased satellite cell activity that is unable to protect and repair myofiber damage.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13395-020-00252-3.

Follistatin-based ligand trap ACE-083 induces localized hypertrophy of skeletal muscle with functional improvement in models of neuromuscular disease

Skeletal muscle is under inhibitory homeostatic regulation by multiple ligands of the transforming growth factor-β (TGFβ) superfamily. Follistatin is a secreted protein that promotes muscle growth and function by sequestering these ligands extracellularly. In the present study, we evaluated the potential of ACE-083 – a locally acting, follistatin-based fusion protein – as a novel therapeutic agent for focal or asymmetric myopathies. Characterization of ACE-083 in vitro revealed its high affinity for heparin and extracellular matrix while surface plasmon resonance and cell-based assays confirmed that ACE-083 binds and potently neutralizes myostatin, activin A, activin B and growth differentiation factor 11 (GDF11). Intramuscular administration of ACE-083 caused localized, dose-dependent hypertrophy of the injected muscle in wild-type mice and mouse models of Charcot-Marie-Tooth disease (CMT) and Duchenne muscular dystrophy, with no evidence of systemic muscle effects or endocrine perturbation. Importantly, ACE-083 also increased the force of isometric contraction in situ by the injected tibialis anterior muscle in wild-type mice and disease models and increased ankle dorsiflexion torque in CMT mice. Our results demonstrate the potential of ACE-083 as a therapeutic agent for patients with CMT, muscular dystrophy and other disorders with focal or asymmetric muscle atrophy or weakness.

Transgenic zebrafish model of DUX4 misexpression reveals a developmental role in FSHD pathogenesis

Facioscapulohumeral dystrophy type 1 (FSHD-1) is the most common autosomal dominant form of muscular dystrophy with a prevalence of ∼1 in 8000 individuals. It is considered a late-onset form of muscular dystrophy and leads to asymmetric muscle weakness in the facial, scapular, trunk and lower extremities. The prevalent hypothesis on disease pathogenesis is explained by misexpression of a germ line, primate-specific transcription factor DUX4-fl (double homeobox 4, full-length isoform) linked to the chromosome 4q35. In vitro and in vivo studies have demonstrated that very low levels of DUX4-fl expression are sufficient to induce an apoptotic and/or lethal phenotype, and therefore modeling of the disease has proved challenging. In this study, we expand upon our previously established injection model of DUX4 misexpression in zebrafish and describe a DUX4-inducible transgenic zebrafish model that better recapitulates the expression pattern and late onset phenotype characteristic of FSHD patients. We show that an induced burst of DUX4 expression during early development results in the onset of FSHD-like phenotypes in adulthood, even when DUX4 is no longer detectable. We also utilize our injection model to study long-term consequences of DUX4 expression in those that fail to show a developmental phenotype. Herein, we introduce a hypothesis that DUX4 expression during developmental stages is sufficient to induce FSHD-like phenotypes in later adulthood. Our findings point to a developmental role of DUX4 misexpression in the pathogenesis of FSHD and should be factored into the design of future therapies.

RNA helicase, DDX27 regulates skeletal muscle growth and regeneration by modulation of translational processes

Gene expression in a tissue-specific context depends on the combined efforts of epigenetic, transcriptional and post-transcriptional processes that lead to the production of specific proteins that are important determinants of cellular identity. Ribosomes are a central component of the protein biosynthesis machinery in cells; however, their regulatory roles in the translational control of gene expression in skeletal muscle remain to be defined. In a genetic screen to identify critical regulators of myogenesis, we identified a DEAD-Box RNA helicase, DDX27, that is required for skeletal muscle growth and regeneration. We demonstrate that DDX27 regulates ribosomal RNA (rRNA) maturation, and thereby the ribosome biogenesis and the translation of specific transcripts during myogenesis. These findings provide insight into the translational regulation of gene expression in myogenesis and suggest novel functions for ribosomes in regulating gene expression in skeletal muscles.

Whole body periodic acceleration is an effective therapy to ameliorate muscular dystrophy in mdx mice

Duchenne muscular dystrophy (DMD) is a genetic disorder caused by the absence of dystrophin in both skeletal and cardiac muscles. This leads to severe muscle degeneration, and dilated cardiomyopathy that produces patient death, which in most cases occurs before the end of the second decade. Several lines of evidence have shown that modulators of nitric oxide (NO) pathway can improve skeletal muscle and cardiac function in the mdx mouse, a mouse model for DMD. Whole body periodic acceleration (pGz) is produced by applying sinusoidal motion to supine humans and in standing conscious rodents in a headward-footward direction using a motion platform. It adds small pulses as a function of movement frequency to the circulation thereby increasing pulsatile shear stress to the vascular endothelium, which in turn increases production of NO. In this study, we examined the potential therapeutic properties of pGz for the treatment of skeletal muscle pathology observed in the mdx mouse. We found that pGz (480 cpm, 8 days, 1 hr per day) decreased intracellular Ca²⁺ and Na⁺ overload, diminished serum levels of creatine kinase (CK) and reduced intracellular accumulation of Evans Blue. Furthermore, pGz increased muscle force generation and expression of both utrophin and the carboxy-terminal PDZ ligand of nNOS (CAPON). Likewise, pGz (120 cpm, 12 h) applied in vitro to skeletal muscle myotubes reduced Ca²⁺ and Na⁺ overload, diminished abnormal sarcolemmal Ca²⁺ entry and increased phosphorylation of endothelial NOS. Overall, this study provides new insights into the potential therapeutic efficacy of pGz as a non-invasive and non-pharmacological approach for the treatment of DMD patients through activation of the NO pathway.

Individual and combined effects of exercise and alendronate on bone mass and strength in ovariectomized rats

Exercise and bisphosphonate therapies increase bone strength by primarily increasing bone formation and reducing resorption, respectively. Based on these different mechanisms of action, it is possible that combined introduction of exercise and bisphosphonate therapies generates greater improvements in bone mass and strength than either intervention alone. The aim of this study was to examine the individual and combined effects of exercise (treadmill running) and bisphosphonate therapy (alendronate [ALN]) on bone mass and strength in ovariectomized (OVX) rats. Seven-month-old virgin female rats were randomly assigned to either a sham-OVX group (n=13) or one of four OVX groups: vehicle-treated cage-control (VEH-CON, n=10); ALN-treated cage-control (ALN-CON, n=13); vehicle-treated plus treadmill running (VEH-RUN, n=13); and ALN-treated plus treadmill running (ALN-RUN, n=13). ALN-treated groups received twice-weekly ALN (0.015 mg/kg), and exercise groups ran on a motorized treadmill at a 5% incline for 60 min/day, 22-24 m/min, 5 days/week. In vivo measurements included dual-energy X-ray absorptiometry (DXA) of whole-body bone mineral content (BMC), and ex vivo measurements included DXA, micro-computed tomography (muCT), and mechanical testing of the femur and L4 vertebrae. After 14 weeks of intervention, exercise and ALN had additive benefits on whole body and proximal femur BMC, cross-sectional area of the L4 vertebrae, and mechanical properties of the mid-shaft femur. In comparison, for total and mid-shaft femur BMC, L4 vertebrae BMC, and mid-shaft femur cortical thickness and area, there were significant exercise and ALN interactions indicating that the two interventions worked in synergy to enhance bone properties. Supporting the contention that ALN and exercise function via distinct mechanisms of action, ALN successfully reduced medullary canal area suggesting it reduced endocortical bone resorption, whereas exercise augmented periosteal perimeter suggesting it stimulated periosteal bone formation. In summary, we found combined treadmill running and ALN to be more beneficial in preventing declines in bone mass and strength following OVX than the introduction of either intervention alone. These data suggest that a comprehensive program of bisphosphonate therapy and weight-bearing exercise may be an effective method for preventing and treating osteoporosis in post-menopausal women.

Cardiorespiratory responses to physical work during and following 17 days of bed rest and spaceflight

To determine the influence of a 17-day exposure to real and simulated spaceflight (SF) on cardiorespiratory function during exercise, four male crewmembers of the STS-78 space shuttle flight and eight male volunteers were studied before, during, and after the 17-day mission and 17 days of −6° head-down-tilt bed rest (BR), respectively. Measurements of oxygen uptake, pulmonary ventilation, and heart rate were made during submaximal cycling 60, 30, and 15 days before the SF liftoff and 12 and 7 days before BR; on SF days 2, 8, and 13 and on BR days 2, 8, and 13; and on days 1, 4, 5, and 8 after return to Earth and on days 3 and 7 after BR. During 15 days before liftoff, day 4 after return, and day 8 after return and all BR testing, each subject completed a continuous exercise test to volitional exhaustion on a semirecumbent (SF) or supine (BR) cycle ergometer to determine the submaximal and maximal cardiorespiratory responses to exercise. The remaining days of the SF testing were limited to a workload corresponding to 85% of the peak pre-SF peak oxygen uptake (V̇o2 peak) workload. Exposure to and recovery from SF and BR induced similar responses to submaximal exercise at 150 W. V̇o2 peak decreased by 10.4% from pre-SF (15 days before liftoff) to day 4 after return and 6.6% from pre-BR to day 3 after return, which was partially (SF: −5.2%) or fully (BR) restored within 1 wk of recovery. Workload corresponding to 85% of the peak pre-SF V̇o2 peak showed a rapid and continued decline throughout the flight (SF day 2, −6.2%; SF day 8, −9.0%), reaching a nadir of −11.3% during testing on SF day 13. During BR, V̇o2 peak also showed a decline from pre-BR (BR day 2, −7.3%; BR day 8, −7.1%; BR day 13, −9.0%). These results suggest that the onset of and recovery from real and simulated microgravity-induced cardiorespiratory deconditioning is relatively rapid, and head-down-tilt BR appears to be an appropriate model of this effect, both during and after SF.

Treadmill validation of an over-ground walking test to predict peak oxygen consumption

The purpose of this study was to determine whether a test developed to predict maximal oxygen consumption (VO2max) during over-ground walking, was similarly valid as a predictor of peak oxygen consumption (VO2) when administered during a 1-mile (1.61 km) treadmill walk. Treadmill walk time, mean heart rate over the last 2 full min of the walk test, age, and body mass were entered into both generalized (GEN Eq.) and gender-specific (GSP Eq.) prediction equations. Overall results indicated a highly significant linear relationship between observed peak VO2 and GEN Eq. predicted values (r = 0.91), a total error (TE) of 5.26 ml.kg-1.min-1 and no significant difference between observed and predicted peak VO2 mean values. The peak VO2 for women (n = 75) was predicted accurately by GSP Eq. (r = 0.85; TE = 4.5 ml.kg-1.min-1), but was slightly overpredicted by GEN Eq. (overall mean difference = 1.4 ml.kg-1.min-1; r = 0.86; TE = 4.56 ml.kg-1.min-1). No significant differences between observed peak VO2 and either GEN Eq. (r = 0.85; TE = 4.3 ml.kg-1.min-1) or GSP Eq. (r = 0.85; TE = 4.8 ml.kg-1.min-1) predicted values were noted for men (n = 48) with peak VO2 values less than or equal to 55 ml.kg-1.min-1. However, both equations significantly underpredicted peak VO2 for the remaining high peak VO2 men (n = 22). In conclusion, the over-ground walking test, when administered on a treadmill, is a valid method of predicting peak VO2 but underpredicts peak VO2 of subjects with observed high peak VO2 values.

Development of a single-stage submaximal treadmill walking test

An equation was developed to estimate maximal oxygen uptake (VO2max, ml.kg-1.min-1) based on a single submaximal stage of a treadmill walking test. Subjects (67 males, 72 females) aged 20-59 yr completed 4-min stages at 0, 5, and 10% grades walking at a constant speed (2.0-4.5 mph) and then performed a VO2max test. Heart rate and respiratory gas exchange variables were measured during the test. Multiple regression analysis (N = 117) to estimate VO2max from the 4-min stage at 5% grade yielded the following model (R2 = 0.86; SEE = 4.85 ml.kg-1.min-1): VO2max = 15.1 + 21.8*SPEED (mph) -0.327*HEART RATE (bpm) -0.263*SPEED*AGE (yr) + 0.00504*HEART RATE*AGE + 5.98*GENDER (0 = Female; 1 = Male). The constant and all coefficients were highly significant (P less than 0.01). To assess the accuracy of the model in a cross-validation group (N = 22), an estimated VO2max value was obtained using the above model. Estimated VO2max then was regressed on observed VO2max yielding the following equation (R2 = 0.92): ESTIMATED VO2max = 0.15 + 1.03*OBSERVED VO2max. The intercept and slope of this equation were not significantly different from 0 and 1, respectively. For 90.9% of the subjects in the cross-validation group, residual scores were within the range of +/- 5 ml.kg-1.min-1. In conclusion, this submaximal walking test based on a single stage of a treadmill protocol provides a valid and time-efficient method for estimating VO2max.